Lightning, which is more common in hotter temperatures, is usually the cause of wildfires that burn the most land. In 2023, scientists at Canada’s natural resources department found that lightning sparked fires that burned 93 percent of the total wildfire area, and the remaining seven percent of the area burned from human causes.

Smoke from wildfires in Canada has spread more than 5,000 miles, reaching parts of Russia and Europe during the past week.

An air quality map shows conditions improving in the U.S. on Friday as smoke from the Canadian wildfires subsides.